一个查询中有多个连续的组Mongodb

以下是我的工作代码:

age_range = DemographicsData.collection.aggregate([
  { '$match' => { video_id: { '$in' => video_ids }}},
  { '$unwind' => '$demographics_labels' },
  { '$group' => { _id: '$demographics_labels.age_range', count: { '$sum' => 1 }}}
])
gender = DemographicsData.collection.aggregate([
  { '$match' => { video_id: { '$in' => video_ids }}},
  { '$unwind' => '$demographics_labels' },
  { '$group' => { _id: '$demographics_labels.gender', count: { '$sum' => 1 }}}
])
ethnicity = DemographicsData.collection.aggregate([
  { '$match' => { video_id: { '$in' => video_ids }}},
  { '$unwind' => '$demographics_labels' },
  { '$group' => { _id: '$demographics_labels.ethnicity', count: { '$sum' => 1 }}}
])
{
  age: age_range,
  gender: gender,
  ethnicity: ethnicity,
}

Rails 3.2.8版

Ruby 1.9.3版